health check changes
health check changes

* change the checking processes to use gen_server:cast instead of exit for
  message passing between it and the node_watcher process for valid return
  values (true/false). Invalid return values are still handled via exit.
* provide functions (resume_health_checks/suspend_health_checks) on
  riak_core_node_watcher to enable/disable all health checks. This will also
  toggle the new healths_enabled flag in the node_watcher's state which is
  used to prevent re-starting checks when a node goes down and comes back up

Kyorai/riak_core